The present invention relates to additives for use in low VOC aqueous coatings. More specifically, the additives comprise a carboxylic acid, an amine and an alcohol; these additives reduce the surface tension when used in water-based systems.
Coatings are typically applied to the interior of metal food and beverage containers to prevent the contents from contacting the metal surface of the container. Contact with certain foods, particularly acidic products, can cause the metal container to corrode. This corrosion results in contamination and deterioration in the appearance and taste of the food or beverage product.
The internal protective coating applied to metal cans is typically one that has low extractibles to avoid contamination of the contents. The coating should also be substantially defect-free, and should possess high resistance to a wide variety of foods and beverages. Good adhesion to metal surfaces is also desired, as is good wetting, to ensure complete coverage to the metal and to protect the metal during baking and forming operations. The high temperatures needed to achieve rapid curing speeds, however, often result in blistering of the coating. Blistering typically occurs as cure temperature passes through the boiling point of water and can result in incomplete or weakened coverage of the can interior.
In addition, after can fabrication the aqueous coating must withstand the relatively stringent temperature and pressure requirements to which cans are subjected during food processing and that will provide the necessary level of corrosion resistance to the cans once filled.
The present invention relates to an additive for low VOC aqueous coatings comprising a carboxylic acid and an amine, in conjunction with an alcohol. The additive can be incorporated into a liquid, aqueous coating, including one that is pigmented. These coatings and methods of making these coatings are also within the scope of the present invention.
The additives of the present invention serve to reduce the surface tension of the aqueous coatings into which they are placed. This results in improved xe2x80x9cwettingxe2x80x9d of the coating on the metal surface. xe2x80x9cWettingxe2x80x9d is a term used in the art to refer to the ability of a coating, particularly a waterborne coating, to cover a substrate with a continuous film, substantially free of defects. The use of compositions comprising the present additives results in a very complete coverage of the metal surface of the can, particularly over washcoat that has splashed inside the cans and at the bead area of the cans, which have historically been spots where adequate coating coverage can be hard to achieve. Better coating coverage correlates with better corrosion resistance. Significantly, the present additives enhance the performance of the coating without having any negative impact on the coating. It is difficult to find additives that do not negatively affect performance of the coating because the requirements for the coating application are so stringent; a very rapid, high temperature cure is employed in coating the cans and the cans are subjected to very high temperatures and pressures after filling. Cans coated with coatings containing the present additives also give excellent enamel rater performance, which confirms that the cans are substantially coated. An added advantage of the present compositions is that they are low in volatile organic content (VOC). Their use is therefore environmentally beneficial.